Guide to Unix/Explanations/Fake Networking Interfaces

The system uses the loopback interface "lo0" to talk to itself. Most systems automatically configure it at boot. Talking-to-oneself is useful for testing servers, and also for any situation where the server and client are on the same computer. (For example, some computer games have a one-player mode where server and client are on the same computer. These games use the "network", but because there is only one computer, they use the loopback interface.)

Use the ifconfig command to check the status of the interface. It probably looks something like:

The IPv4 address of your computer is "127.0.0.1". This is in addition to any address that it has on an actual network. In fact, every computer calls itself "127.0.0.1", so it is impossible to use this address to communicate between computers. (Some jokes involve a person who is given "127.0.0.1" as an attack target and proceeds to destroy their own computer.)

If your system is running an IPv6 stack, then your IPv6 loopback address is "::1". The computer should be able to use IPv6 to communicate with itself, even if it has no connection to an actual IPv6 network.

You probably have no problem pinging the interface with ping or ping6. We use "ping -c3" and "ping6 -c3" to ping 3 times; otherwise it would ping every second until you interrupted with ^C (CTRL+C).